The present invention relates to a comparator for comparing an input signal with a reference voltage. More specifically, the present invention is directed to a comparator for comparing an analog signal having a wide range of an input voltage, to a LSI test system for testing electric characteristics of an IC, and also to an apparatus which requires a comparison/judgement of voltage levels.
Conventionally, in this sort of the comparator, an allowable input voltage range for an input signal relative to a reference voltage is determined based upon a breakdown voltage VEBO between a base and an emitter of a differential transistor pair (namely, reverse-voltage application range of VBE). When a reverse voltage applied across a base and an emitter of a transistor is increased, a current amplification factor hFE is lowered. In general, a reverse voltage between a base and an emitter of a bipolar transistor under which no reduction of a current amplification factor hFE is caused, is on the order of about 2 to 3 V, namely low voltage. A maximum voltage which can be applied across a base and an emitter of a transistor without lowing the factor hFE will be referred to as a base-emitter reverse withstanding voltage VR hereinafter.
In other words, an input voltage range which does not deteriorate characteristics of a comparator circuit is determined by the base-emitter reverse withstanding voltage VR of a transistor, and thus, there is such a problem that a voltage difference across an input table input voltage and a reference voltage is small.
FIG. 4 indicates an example of the most generic differential type comparator circuit with employment of a differential transistor pair. An input signal Vin is inputted to an input terminal 20, and a reference voltage Vref is entered to another input terminal 21 of this differential type comparator circuit. Both a transistor 1N and another transistor 2N are turned ON/OFF in response to a high/low relationship of voltage levels of the input signal Vin and the reference voltage Vref. A current I0 of a current source 6 flows through either a resistor 8 or a resistor 9, thereby producing a voltage drop and thus outputting a comparison judgement level (H/L) to output terminals 22 and 23.
FIG. 5A to FIG. 5C represent conditions of changes in base-emitter voltages VBE of the transistors 1N and 2N in such a case that an input signal Vin 20 is changed with time, and is compared with a certain reference voltage Vref 21. Until the present time reaches time t1 (in case of input signal Vin 20 less than reference voltage Vref 21), the transistor 1N is turned OFF and the transistor 2N is turned ON. When the present time exceeds the time t1 (in case that input signal Vin 20 greater than reference voltage Vref 21), the transistor 1N is turned ON and the transistor 2N is turned OFF. As to a reverse voltage applied across a base and an emitter of a transistor set under OFF state, assuming now that a voltage across a base and an emitter of such a transistor set under ON state is equal to VBE(ON), a voltage of input signal Vin 20xe2x88x92reference voltage Vref 21xe2x88x92VBE(ON) is applied. If the ON-voltage VBE(ON) between the base and the emitter is neglected, then a difference between the input signal Vin 20 and the reference voltage Vref 21 is applied as the reverse voltage across the base and the emitter of the transistor set to the OFF state (see 91 of FIG. 5B and 92 of FIG. 5C). While a base-emitter reverse withstanding voltage VR of a bipolar transistor is about 2 to 3 V, namely a low voltage, an input voltage range which does not deteriorate the characteristic of the comparator is determined based upon the reverse withstanding voltages VR of the transistors 1N and 2N. As a result, there is such a drawback that the input signal can be compared with the reference voltage only when a difference between the input signal and the reference voltage is 2 to 3 V in maximum.
Conventionally, JP-A-2-69016 has described a technique of such a comparator for aiming an enlargement of a voltage range for an input signal. FIG. 6 shows a similar circuit diagram to this conventional comparator. In this comparator circuit, a voltage dividing circuit 37 constituted by a plurality of resistors is inserted into a prestage of a comparator 103. FIG. 7A to FIG. 7D show voltage levels Vinxe2x80x2 200, and Vrefxe2x80x2 211 inputted to the comparator 103, and conditions of changes in voltages VBE applied across the bases and the emitters of the transistors 1N and 2N in the case that the input signal Vin 20 is changed with time, and is compared with a certain constant reference voltage Vref 21. As indicated in FIG. 7B, as the voltages Vinxe2x80x2 200 and Vrefxe2x80x2 211 inputted to the comparator 103, such voltages are entered which are produced by reducing the original input signal Vin 20 and the original reference voltage Vref 21 by a resistance ratio (voltage dividing ratio) of the voltage dividing circuit 37 provided at the prestage of this comparator 103. Since the resistor values of the voltage dividing circuit 37 are determined in such a manner that the resistor values do not exceed reverse withstanding voltages VR between the bases and the emitters of the transistors 1N and 2N in consideration of the input voltage range of the input signal Vin and the reference voltage Vref, the enlargement of the input voltage range is realized.
With the arrangement of the voltage dividing circuit 37 at the prestage of the comparator 103, however, a low-pass filter (LPF) is undesirably constituted by stray capacitances of both the transistors 1N and 2N, and both the resistors 95 and 96 of the voltage dividing circuit 37. As a consequence, there is such a problem that high frequency components of the input signal are cut off by the low-pass filter LPF, and thus, the input band width of the comparator 103 is restricted.
The present invention has been made to solve the above-described problems, and therefore, has an object to provide a comparator capable of enlarging a voltage range of an input signal, and capable of directly comparing/judging a voltage level of the input signal with respect to a voltage level of a reference voltage.
Another object of the present invention is to provide a comparator which is especially suitable for a LSI test system for testing ICs.
To achieve the above-described objects, typical inventive comparators among comparators according to the present invention are as follows:
According to one aspect of the invention, a comparator is comprised of: a pair of transistors which are continuously brought into ON states; wherein: switch means constructed of a diode pair, for switching a current path in response to a high/low relationship between a voltage level of an input signal and a voltage level of a reference voltage; and means for converting the current into a voltage level are provided between emitter terminals of the transistor pair.
According to another aspect of the invention, a comparator is comprised of: an input terminal having a diode; a differential transistor pair in which the transistors are turned ON/OFF in response to a high/low relationship between a voltage level of an input signal and a voltage level of a reference voltage; a circuit constituted by a diode between base terminals of the differential transistor pair; and means for converting the current which is switched by the differential transistor pair into a voltage level.
According to still another of aspect of the invention, a comparator is comprised of: a differential transistor pair which is turned ON/OFF in response to a high/low relationship between a voltage level of an input signal and a voltage level of a reference voltage; diodes each provided at the emitter terminal of the differential transistor pair and diodes each provided between the emitter and the base of the differential transistor pair; and means for converting a current which is switched by the differential transistor pair into a voltage level.
According to still further aspect of the invention, a comparator is comprised of: a diode bridge circuit provided at an one input terminal; a differential transistor pair which is turned ON/OFF in response to a high/low relationship between a voltage level of an input signal and a voltage level of a reference voltage; a push-pull circuit provided between base terminals of the differential transistor pair; and means for converting a current which is switched by the differential transistor pair into a voltage level.